It is known that traditional quadrature FM detectors are used in FM receiver IF stages to demodulate two-tone and multilevel FSK signalling. This method is successful because a quadrature FM demodulator detects the difference between an input IF signal frequency and a fixed center frequency tuned by a bandpass filter.
It is also known that a frequency and phase-locked loop (FPLL) may be used to detect the difference between two signals--the input signal and the output of a VCO. This new type of frequency difference detector is a quadrature FM detector that has had a "bandpass-to-lowpass" transformation performed on it.
The idea of configuring an FM demodulator in a "zero IF" form is to utilize the "bandpass-to-lowpass" transformation to the advantage that "drift" of the bandpass center tuning filter can be eliminated. As a result, the discriminator's characteristic "S" curve is now solely dependent on the phase characteristic of the low pass filter, which is now designed at "baseband" frequencies. Proper design of this LPF can produce the required "S" curve. Essentially, the whole FSK demodulator (two-tone or multilevel) could be designed as a fast-acquisition FPLL, but it might be easier to use just the frequency difference detector to create a zero-IF signal, ready for demodulation by baseband components operating in an open-loop mode.
As a result, it has been proposed that the VCO be replaced with a crystal-controlled oscillator. This double-mixer concept will produce 2 signals that are in phase quadrature, but converted down to baseband.
One advantage of using baseband signals that are time-delayed to demodulate an FSK signal is that the time-delay element, to wit, the LPF, may be replaced with a digital equivalent. This digital equivalent may be implemented as a tapped delay line with selectable taps. If this is done directly at the IF frequency (traditional IFs could be in the low VHF band), the period of the IF waveform would be only a few nanoseconds. For example, the period of a 70 MHz IF signal is only 14.2857 nanoseconds. The time-delay FSK demodulation method requires a 90.degree. phase difference between 2 modulated signal paths to produce a maximum demodulated output amplitude at the maximum frequency deviation. Hence, these 2 signals are only "out of phase" by 3.5714 nanoseconds. As a result, it is difficult to achieve multiple tap positions over only 3.57 nanoseconds total delay. Obviously, it would be desirable to provide an improved multilevel FSK demodulator.
Traditional quadrature FM discriminators utilize a phase shift network that is tuned to the center of the IF carrier. This phase shift network typically consists of a resonant circuit, such as a parallel coil and capacitor. To obtain equal amplitude excursions of the demodulated signals the network must be tuned to the exact frequency of the IF carrier. A DC offset will result if the network becomes tuned other than to the center of the incoming signal. Thus, component tolerances for the network are very critical.